The present invention relates to steel elements which can be used for reinforcing vulcanized elastomeric articles based on natural or synthetic rubber, such as tires, belts, hoses, straps and like products.
More particularly, it relates to rubber adherable steel elements for reinforcing rubber articles which are vulcanizable with sulphur, such as e.g. vehicle tires. These reinforcing elements are generally covered with a brass coating, thereby providing better adhesion to the rubber.
The term steel reinforcing elements as used herein is intended to be generic to all steel products suitable for strengthening rubber articles, including wires, filaments, strands, cables, tire cords, steel plates, shaped wire products and combinations thereof without being limited thereto.
The term steel refers to what is commonly known as high carbon steel, i.e. iron-carbon alloys containing from 0.4 to 1.4% carbon, usually from 0.6 to 1% C, and which may contain additional alloying elements in varying amounts.
The term brass refers to an alloy substantially of copper and zinc, the composition of which can also include other metals in varying lesser amounts. The copper content of a rubber adherable brass composition can range from 50 to 99% by weight, but in the majority of cases, such as e.g. in bonding steel cord to rubber components for tires, a copper content ranging from 55 to 75% is now regarded as being most suitable by those skilled in the art.
Hence, the wide-spread practice of vulcanization of rubber onto a brass plated metal substrate is now extensively applied in manufacturing steel reinforced rubber articles, and in particular the use of brassed steel cord for tire materials is well known.
Steel cord for use in tire applications is normally made by twisting or cabling together brassplated high-carbon steel wires, drawn to a filament diameter of from about 0.10 to 0.50 mm. The brass alloy coating usually comprises 60 to 75% Cu and 40 to 25% Zn; the plating thickness may range from 0.05 to 0.50 .mu.m, preferably from 0.10 to 0.35 .mu.m. In practice, the specific composition and thickness of the brass alloy coating on the wire are restricted by the adhesion requirements for a given rubber compound and by wire manufacturing considerations. Hence, brass composition and plating thickness are optimized in each case to obtain maximum "initial" adhesion (i.e. just after vulcanization) and to afford good wire drawability, given the large deformation and friction imposed on the coating during the final wire manufacturing steps. For this purpose it is advisable to have a brass alloy with homogeneous .alpha.-structure, i.e. a composition which is substantially free of the .beta.-phase (a hard and less deformable crystal type) which gradually appears below 62-63 % Cu, and even from below 65% Cu in less homogeneous Cu-Zn alloy deposits.
At present, the dual requirement of securing an adequate initial rubber to steel adhesion and of facilitating the drawing of the wire is reasonably well solved by known brass coatings and forms part of the state of the art. However, maintaining a sufficiently high post-cured adhesion level during the service life of the rubber article, e.g. during the running life of the tire, is still a major problem in the industry. It has been observed that moisture is generally very detrimental to the adhesion between the brass plated steel reinforcing element and the rubber article. Variation of water content in the unvulcanized rubber compound, for instance, is already known to be a problem. Of even greater importance is the effect of humidity (water pick-up) and heat after curing on degradation of the adhesive bond, especially during the service life of the steel cord reinforced rubber article, e.g. a tire subject to harsh driving conditions. In fact, it has been acknowledged by tire specialists and cord manufacturers that adhesion retention is severely affected by humidity ageing and related effects causing degradation, involving heat corrosion, and that a high initial adhesion level achieved for a given brass coating is no guarantee of maintaining a satisfactory adhesion level during the lifetime of e.g. a tire. Seeking optimization of the brass coating, in particular a solution to the humidity ageing problems for a given rubber compound, results in most cases in the use of thinner brass layers with low copper content. Unfortunately, this solution suffers from some practical difficulties, such as e.g. a lower initial adhesion, corrosion problems and poor wire drawability, especially when the brass composition has a copper content below 65%.
In the past, a number of attempts have been made to solve the difficulties posed by the presence of moisture and by the simultaneous action of humidity and heat.
These trials include the modification of the brass coating by alloying Cu-Zn with different metals, such as Co, Ni, Pb, Sn and even Fe, so as to obtain a homogeneous ternary brass alloy. Other proposals include the deposition of a protective metal layer of Ni or Zn between the steel substrate and the brass coating, the treatment of the brass surface with various chemicals to clean the brass surface in depth and/or modification of the outermost layer with adhesion promoters and/or corrosion inhibitors. In addition the deposition of a thin corrosion resistant metal film of zinc and the alloying of the brass surface with cobalt have been proposed.
A number of these methods are described in the following prior art documents:
U.S. Pat. No. 3,858,635 proposes the use of Sn, Pb and the like.
U.S. Pat. No. 3,749,558 describes the use of Cu-Ni and Cu-Ni Zn coatings
U.S. Pat. No. 4,299,640 proposes the treatment of the brass surface with certain amino carboxylic acids and their salts.
U.K. Pat. No. 2,011,501A describes the use of ternary brass alloy coatings containing Cu, Zn and Co.
U.S. Pat. No. 2,076,320 describes brass-coated metal objects with a high cobalt concentration gradient on their surface.
U.S. Pat. No. 4,143,209 describes a process for plating a brass-coated wire with a zinc layer.
U.S. Pat. No. 4,446,198 reveals a ternary brass alloy coating containing Cu, Zn and Fe.
Some of these attempts have been successful in solving one or another specific aspect of the brass to rubber adhesion problem. However, there still remain deficiencies and uncertainties.
In practice, the use of ternary brass alloy coatings is less reliable because of more frequent compositional fluctuations, a complex process, and the difficulty in maintaining close tolerances over a long manufacturing period. When using low melting-point metals on top of the brassed wire, it is found that they migrate to a variable degree in the brass layer during the wire drawing process. Cobalt deposits are expensive, less deformable and sometimes detrimental, depending on vulcanization conditions and rubber type.
Thus, none of the proposed methods to prevent loss of rubber adhesion to conventionally prepared wires or to wires with modified brass coating are sufficiently successful to find widespread commercial use. They are often unable to tackle the stated problem in its entirety, i.e. the pursuit of adhesion retention under varying working conditions when combining brass coatings and rubber compounds of different origin, preferably without sacrificing too much in terms of manufacturing reliability and wire cost.